Large Solar Flares. Albert Y. Shih NASA/GSFC 2014 Oct 21

Transcription

1 Large Solar Flares Albert Y. Shih NASA/GSFC 2014 Oct 21

2 The Carrington event 1859 Sep 1: First observation of a flare Compared to other flares (Carrington 1859) (Cliver& Dietrich 2013)

3 2014 Oct 19, X1.1 flare Active region Å, 193Å, 171Å (SolarMonitor.org) (spaceweather.com)

4 Definition of GOESclass X1.1

5 Totalsolar irradiance (TSI) ~100 ppm natural fluctuations of TSI p-mode oscillations convection noise SORCE/TIM Only the largest flares can be clearly seen in TSI 2003 Oct 28, X17 (Woods et al. 2006)

6 Observables Flare phases Impulsive rise Gradual decay Photons Across all energies Neuperteffect: nonthermalsignature matches the derivative of thermal signature Particles Electrons and ions Neutrons Energetic neutral atoms M Å Å 6 12 kev kev kev kev derivative of 1 8 Å derivative of Å (courtesy B. Dennis)

7 Multi-mission view of a solar eruptive event

8 Solar eruptive events (SEEs) Solar flare Impulsive release of energy at the Sun Broadband emission Solar eruptive event Flare Associated coronal mass ejection (CME) Magnetic field contains the energy for the flare Stores for long times Releases up to ergs on the timescale of thousands of seconds (Holman 2012)

9 Magnetic reconnection 2D reconnection Observing reconnection (Wikipedia) (Su et al. 2013)

10 3D slip-running reconnection Flux-rope growth Flare-loop formation (Janvier et al. 2013)

11 Solar cycles (Wikipedia)

12 Large flares over solar cycles Solar cycles 21, 22, and 23 Sunspot number (blue) X-class flares (black) Flares with detected neutron-capture line emission (red) Significant acceleration of >~20 MeV protons Large flares occur more during declines

13 Solar flare distribution Direct bolometric measurements (black) ~ ergs Extrapolation with maximum of ergs (dashed) Scaled from other wavelengths (red) Power-law index of ~2.3 Compared to active Sunlike stars (black for G, bluefor K) Scaled by X-ray luminosity (Schrijver et al. 2012)

14 Energetics of six largest SEEs (Emslie et al. 2012)

15 SEE energetics survey results I E GOES ~ 0.05 E T-rad E T-rad ~ 2.7 E peak E peak ~ 0.11 E part E T-rad ~ 0.31 E part

16 SEE energetics survey results II E ion ~ 0.34 E elec E SEP ~ 0.04 E CME-KE E SEP ~ 0.27 E ion E bol ~ 0.06 E mag

17 SEE energetics survey results III E CME ~ 0.19 E free E bol ~ 0.35 E CME E part ~ 0.71 E bol E T-rad ~ 0.21 E bol

18 Radiated energy from the chromosphere 2011 Feb 15, X2.2 (Milligan et al. 2014)

19 Gamma rays SEE corona n (cm -3 ) electrons protons, alphas, heavy ions bremsstrahlung nuclear de-excitation, positron-annihilation, pion-decay emission neutrons Thermalization time delay of ~ 100 seconds spatial separation of < 1 arcsec neutron-capture photosphere

20 High-energy flare spectroscopy thermal plasma emission nonthermal bremsstrahlung ion-associated emission Thermal plasma emission Plasma temperature Plasma abundances Relativistic bremsstrahlung continuum from electrons Electron energy spectrum Electron angular distribution Nuclear de-excitation lines Ambient abundances Accelerated abundances Shape of the ion energy spectrum (~3 10 MeV) Neutron-capture line Higher-energy accelerated ions (~20 MeV) Positron-annihilation line Ambient conditions such as temperature

21 Ion and electron acceleration Direct proportionality >300 kevbremsstrahlung from relativistic electrons Neutron-capture line from >~30 MeV ions Separate RHESSIflares, observed completely (circle) or only partially (triangle) Dotted lines are factors of 2 from the best-fit line, and almost all flares fall within 1 σ of spread (Shih et al. 2009)

22 Gamma-ray line images 35-arcsecond RHESSI gamma-ray angular resolution (FWHM) 2003 Oct 28, X17 Resolved gamma-ray line footpoints Significant shift of ionassociated footpointsversus electron-associated footpoints Also observed in one other event: 2002 Jul 23, X4.8 Gamma-ray line centroid displaced from electronassociated emission by >~30 arcseconds RHESSI 2003 Oct 28 X17 TRACE 195Å (image) electron-associated ion-associated (Hurford et al. 2006)

23 Implications for particle acceleration Correlation of fluencesindicates a connected acceleration process Relativistic electrons >~20 MeV protons Spatial separation indicates a difference in acceleration and/or transport processes Gradient and curvature drifts are inadequate by >~ two orders of magnitude

24 Nuclear de-excitation lines Fe Mg Ne Si C O Predicted de-excitation lines for different spectral indices, with α/p=0.1, coronal accelerated abundances, photospheric ambient abundances, and θ=30.

25 Gamma-ray line spectroscopy Forward-fit to nuclearline templates (see Murphy et al. 2009) 2003 Nov 2, X8.3 Divided into two intervals Spectral index changes from 4.5±0.4 to 5.2±0.3 Alpha/proton ratio stays in the range

26 Pion-decay emission Pion production/decay >~300 MeV protons π 0 2γ π ± e ± + ν e Results in photon continuum Broad bump around ~70 MeV Bremsstrahlung from electrons and positrons 2010 Jun 12, M2 Combined spectrum from GBM and LAT on Fermi pion-decay model bremsstrahlung model (Ackermann et al. 2012)

27 Fermi/LATlong-duration flares I 2011 Mar 7, M3.7 Source localization (Ackermann et al. 2014)

28 Fermi/LATlong-duration flares II 2012 Mar 7, X5.4+X1.3 Type of emission? Pion decay from protons Bremsstrahlung from electrons Type of acceleration? Continuous Trap + precipitation Source of acceleration? Corona (flare) CME-associated shock (Ajello et al. 2014)

29 Super-hot flare plasmas Past measurements not sensitive to plasma temperatures >~30 MK X-class flares have such super-hot plasmas Requires >~100 G coronal magnetic fields to contain the energy density (Caspi et al. 2014)

30 EUV late-phase emission Sun-as-a-star measurements of EUV emission lines SDO/EVE nm at 0.1 nm resolution Impulsive phase Transition-region emission(he II 304Å) Gradual phase Hot corona (>~5 MK) Coronal dimming Cool corona Late phase Warm corona Loops high above the flare site (Woods et al. 2011)

31 Lyman-alpha emission Like TSI, low contrast over full Sun PROBA2/LYRA 2010 Feb 8, M2 0.6% increase in Ly-α Profile delayed compared to SXR emission (Kretzschmar et al. 2012)

32 White-light emission coincidence with HXR footpoints Hinode/SOT 2006 Dec 14, X1.5 (Watanabe et al. 2010) SDO/HMI 2011 Feb 24, M3.5 (Martinez Oliveros et al. 2012)

33 Sub-mm radiation (Krucker et al. 2013) Gyrosynchrotron radiation peaking at ~10 GHz is expected Flare emission can show positive slopes at submm frequencies! Solar Submillimeter-wave Telescope (SST) in Argentina 212 GHz and 405 GHz Indicative of a THz component?

34 Terahertz radiation 2012 Mar 13, M8 Spectrum from radio to IR (Kaufmann et al. 2013)

35 Radio imaging spectroscopy Type III radio burst Signature of coronal electron beams Frequency proportional to square root of density Decimeter wavelengths: type IIIdm Upgraded VLA Confirmation of spatial connection HXR footpointsfrom downward-going electrons Type IIIdmburst from upward-going electrons (Chen et al. 2013)

36 Energetic neutral atoms (ENAs) ENAs produced as energetic protons charge exchange with ambient nuclei Retain energy of original energetic particle No scattering by magnetic field Source of original energetic particles CME-driven shock Escaping directly from flare, but ENAs will likely re-ionize

37 ENA observation 2006 Dec 5 solar eruptive event (X9 flare) LET instruments on the two STEREOspacecraft (Aand B) MeV particles Cannot directly distinguish ENAs from ions ENA evidence Direction of arrival from Sun, not local magnetic field (marked with B ) Arrives ahead of SEP population that diffused across field lines (Mewaldt et al. 2009)

38 ENA measurements Time of flight coincides with solar flare ENA spectral index of ~2.5

39 ENA simulations 1800 km/s CME-driven shock Assume particles effectively trapped downstream of the shock Simulate charge exchange with H, C 4+, and O 6+ ENA behavior depends on energy >~100 kevenas originate below 5 R sun with sharp time profile <~20 kevenas originate above 10 R sun with flat-top time profile Simulated spectrum a factor of 2 greater than Mewaldtet al. (2009) observation (Wang et al. 2014, ApJL)

40 Summary Solar flares release up to ~10 33 ergs Limited by active-region size and efficiency of magneticenergy release Tens of percent in accelerated particles, roughly equipartitioned between electrons and ions Plasma temperatures up to ~50 MK Some emission processes still not well understood Long-duration >100 MeV gamma-ray emission Sub-mm emission with a possible THz component White-light emission